NSAIDs — Mechanisms, Pharmacokinetics, and COX Selectivity

All oral NSAIDs are well absorbed from the gastrointestinal tract. Enteric-coated formulations delay absorption but do not reduce gastric toxicity, because NSAID-induced gastropathy is a systemic effect of prostaglandin suppression, not a local effect of direct mucosal contact. NSAIDs are among the most highly protein-bound drugs in clinical use — nearly all circulating drug is bound to albumin, leaving a very small free fraction that is pharmacologically active. In patients with hypoalbuminemia, this free fraction increases unpredictably.

CYP2C9 metabolizes most clinically used NSAIDs. Inhibitors of CYP2C9 — including fluconazole and amiodarone — reduce NSAID clearance and raise plasma concentrations, increasing toxicity risk. CYP2C9 inducers such as rifampin accelerate clearance and may reduce efficacy. Half-life varies widely across the class and is the primary determinant of dosing interval: short-acting agents (ibuprofen, diclofenac, ketorolac) require more frequent dosing; long-acting agents (naproxen, piroxicam, meloxicam) allow once- or twice-daily dosing but have slower offset when adverse effects occur.

NSAIDs span a continuous spectrum of COX selectivity. Nonselective NSAIDs — including ibuprofen, naproxen, indomethacin, and ketorolac — inhibit both COX-1 and COX-2 at similar concentrations. COX-2 preferential agents — diclofenac and meloxicam — inhibit COX-2 at lower concentrations than COX-1 but are not fully selective. Celecoxib is the only selective COX-2 inhibitor currently marketed in the United States, following the withdrawal of rofecoxib (2004) and valdecoxib (2005) due to cardiovascular harm.

Ibuprofen is the most widely used over-the-counter NSAID. It inhibits COX reversibly and has a short half-life, giving it a rapid offset. One clinically important interaction: ibuprofen can competitively block aspirin's access to platelet COX-1, preventing the irreversible acetylation that underlies aspirin's antiplatelet effect. Patients on aspirin for cardiovascular protection should take aspirin at least 30 minutes before ibuprofen, or substitute naproxen for ibuprofen.

Naproxen has the most favorable cardiovascular risk profile among nonselective NSAIDs, attributed to its long half-life producing sustained COX-1 inhibition that provides a partial antiplatelet effect. It is the preferred NSAID in patients with cardiovascular risk factors when an NSAID cannot be avoided.

Indomethacin is one of the most potent nonselective COX inhibitors. It is first-line for acute crystal arthropathy (gout, pseudogout) and is used intravenously in neonates to close a hemodynamically significant patent ductus arteriosus by blocking the prostaglandin-dependent ductal patency mechanism. Its high rate of central nervous system side effects (headache, dizziness, cognitive changes) limits long-term use.

Ketorolac is the only NSAID available for parenteral (intramuscular and intravenous) administration, giving it a role in acute postoperative and trauma pain when oral dosing is not possible. Its analgesic potency is comparable to moderate opioid doses. Potent nonselective COX inhibition makes its renal and gastrointestinal toxicity significant; current labeling limits use to a maximum of five days.

Celecoxib causes significantly less gastric mucosal injury than nonselective agents. Its gastrointestinal benefit is attenuated when patients also take low-dose aspirin, because aspirin suppresses COX-1 in the gastric mucosa and negates the gastroprotective advantage of COX-2 selectivity. Celecoxib is a moderate inhibitor of CYP2D6 (cytochrome P450 2D6), which is relevant when co-prescribed with CYP2D6 substrates such as metoprolol and codeine.

Diclofenac is COX-2 preferential in vivo and carries a distinct hepatotoxicity signal: transaminase elevations occur in a meaningful proportion of patients on prolonged therapy, and clinically significant liver injury, while uncommon, is more frequent than with other NSAIDs. Liver function monitoring is appropriate during extended use.

PGE2 and PGI2 produced by gastric mucosal COX-1 stimulate mucus and bicarbonate secretion, maintain mucosal blood flow, and inhibit parietal cell acid secretion. NSAID-mediated COX-1 suppression simultaneously impairs all four components of mucosal defense. The gastric mucosa becomes vulnerable to injury from acid, bile salts, and Helicobacter pylori. This mechanism is systemic: enteric-coated and parenteral NSAIDs cause equivalent mucosal injury to plain oral formulations because prostaglandin suppression in the mucosa is driven by the drug in the bloodstream, not by local contact.

Patients at high gastrointestinal risk (prior ulcer or gastrointestinal bleed, age above 65, concurrent anticoagulant or corticosteroid use, high-dose NSAID) should receive either a selective COX-2 inhibitor (celecoxib) or a nonselective NSAID with PPI co-therapy. PPIs reduce the relative risk of NSAID-associated ulcers by approximately 75% and are preferred over misoprostol because of better tolerability. Testing for and eradicating Helicobacter pylori before starting long-term NSAID therapy further reduces risk.

Cardiovascular risk from NSAIDs is proportional to COX-2 selectivity and dose. Selective COX-2 inhibitors suppress endothelial PGI2 while leaving platelet TXA2 intact, creating a prothrombotic state. High-dose diclofenac and ibuprofen carry cardiovascular risk comparable to selective COX-2 inhibitors. Naproxen has the most favorable cardiovascular profile among commonly used NSAIDs and is preferred when an oral NSAID is required in a patient with cardiovascular risk factors.

Renal toxicity is class-wide, independent of COX selectivity, because both COX-1 and COX-2 contribute to renal prostaglandin synthesis. In euvolemic healthy patients, NSAIDs cause minimal reduction in glomerular filtration. In physiologically stressed states — heart failure, cirrhosis, volume depletion, chronic kidney disease — the kidney depends on prostaglandin-mediated vasodilation to maintain perfusion. NSAID-mediated suppression of these prostaglandins in stressed patients can precipitate acute kidney injury rapidly. NSAIDs also cause sodium and water retention, can induce hyperkalemia, and blunt the effect of antihypertensive drugs.

NSAIDs should be avoided in patients with an eGFR below 30 mL per minute per 1.73 m², decompensated heart failure, hepatic cirrhosis with ascites, and in the third trimester of pregnancy (risk of premature closure of the ductus arteriosus).

The combination of an NSAID, a renin-angiotensin-aldosterone system inhibitor (ACE inhibitor or ARB), and a diuretic — the "triple whammy" — carries a markedly elevated risk of acute kidney injury through additive reduction of glomerular filtration and renal perfusion. This combination should be avoided; when unavoidable, renal function and electrolytes must be monitored closely within one to two weeks of initiation.

NSAIDs raise lithium plasma concentrations by reducing renal prostaglandin synthesis and thereby reducing renal lithium clearance. Because lithium has a narrow therapeutic index, even modest increases in concentration can cause toxicity. Lithium levels should be checked within five to seven days of starting or stopping an NSAID in a patient taking lithium.

At oncology doses of methotrexate (above 15 mg per week), NSAIDs impair renal methotrexate excretion and should not be used within 24 hours of a methotrexate infusion. NSAIDs combined with SSRIs increase the risk of upper gastrointestinal bleeding beyond either drug alone because SSRIs deplete platelet serotonin, impairing platelet activation, while NSAIDs suppress TXA2; the combination should trigger PPI co-prescription. NSAIDs combined with warfarin or direct oral anticoagulants multiply gastrointestinal bleeding risk through both mucosal injury and platelet dysfunction.

AERD affects approximately 10 to 20% of adults with asthma, rising to 30% in those with nasal polyps. Any COX-1 inhibiting NSAID can trigger severe bronchoconstriction in these patients through leukotriene shunting. Celecoxib does not inhibit mucosal COX-1 at therapeutic doses and does not trigger this reaction; it is the preferred NSAID in patients with confirmed or suspected AERD. Acetaminophen at standard doses is also generally safe in AERD.

In elderly patients, NSAIDs carry disproportionate risk: reduced renal reserve, diminished gastric mucosal regenerative capacity, higher prevalence of polypharmacy, and increased sensitivity to central nervous system effects of indomethacin. The American Geriatrics Society recommends avoiding oral nonselective NSAIDs in patients 65 and older when alternatives are available. Topical diclofenac gel provides effective local analgesia for hand and knee osteoarthritis with minimal systemic exposure, avoiding most systemic risks.